Oxygen is a gas having high industrial interest because it finds multiple applications in various technical fields: production of steel, glass or paper, medicine, metal welding, combustion or pollution control, for example.
One of the techniques used at present to produce oxygen is the "PSA" (Pressure Swing Adsorption) technique. In the context of the invention, there is meant by PSA processes, not only the PSA processes as such, but also similar processes, such as VSA (Vacuum Swing Adsorption) or MPSA (Mixed Pressure Swing Adsorption) processes.
According to this PSA technique, the oxygen contained in a gaseous mixture comprising essentially oxygen and nitrogen, such as air, is separated from said gaseous mixture by adsorption of nitrogen on a material that preferentially adsorbs nitrogen, said adsorption of the nitrogen being carried out by pressure variation applied in the separation region containing said adsorbent material; the oxygen that is not adsorbed or is little adsorbed is recovered at the outlet of said separation zone.
Such PSA processes have already been described many times in the prior art. Generally speaking, a PSA process always comprises:
an adsorption step selective to nitrogen, on an adsorbent material, at an adsorbent pressure called "high pressure"; PA1 a desorption step of the nitrogen trapped by the adsorbent, at a desorption pressure lower than the adsorption pressure, called a "low pressure"; PA1 a repressurization step of the separation zone comprising the adsorbent, by passage from low pressure to high pressure; and the produced oxygen being recovered during the adsorption phase of the nitrogen. PA1 depends essentially only on the mean granulometry of the adsorbent particles used, particularly the diameter when the adsorbent is in the form of balls, PA1 takes account of the geometric constraints of the dead volumes, PA1 does not have a negative influence on the energy consumption of the PSA process. PA1 the high pressure of adsorption is comprised between 10.sup.5 Pa and 10.sup.6 Pa, PA1 the feed temperature (T.sub.feed) is comprised between 10.degree. C. and 60.degree. C., preferably between 25.degree. C. and 45.degree. C., PA1 the gas flow to be separated is air, PA1 the adsorbent material is selected from zeolites of type X or A, and, preferably, said zeolite comprises at least 50% Alo.sub.2 associated with cations selected from the group comprised by the cations calcium, lithium, zinc, copper, manganese, magnesium, nickel or any alkali or alkaline earth metal, PA1 the cycle time is less than 120 seconds, preferably less than 100 seconds, PA1 e is comprised between 0.1 m and 2 m, preferably between 0.2 m and 1.5 m, PA1 d is comprised between 0.1 mm and 5 mm, preferably between 0.2 mm and 3 mm, PA1 the adsorbent is in shapes substantially spherical, ovoidal, oval, rod shaped or the like. PA1 upstream of the adsorbent bed, that is to say on the feed side, there exists an incompressible dead volume of thickness a (VM supply in FIG. 1) representing particularly the space separating the first isolation valve for supplying the adsorbent and the beginning of the adsorbent bed, the bed of particles of a drying material located upstream of the adsorbent bed, the system of distribution of supply gas (air) in the adsorber. PA1 downstream of the adsorbent bed, which is to say on the production side, there also exists an incompressible dead volume of a thickness b (VM production in FIG. 1) representing particularly the space separating the first shut off valve for production of the adsorber at the end of the adsorbent bed, the system of production gas recover (oxygen).
From this, it is easy to understand that the efficiency of separation of the gaseous mixture depends on numerous parameters, such as the high pressure, the low pressure, the type of adsorbent material, and the affinity of the latter for the components to be separated, the composition of the gaseous mixture to be separated, the temperature of adsorption of the mixture to be separated, the size and shape of the adsorbent particles, the composition of these particles, the thickness of the adsorption bed and the temperature gradient established within said bed, for example.
Until now, no overall regulating law has however been able to be determined, because it is very difficult to relate these various parameters to each other.
In particular, although it is known that the thickness of the adsorbent bed, the granulometry of the adsorbent and the dead volumes have more than a negligible influence on the performances of the PSA unit, particularly as to the pressure drop, no method for selection of the thickness of the bed according to the granulometry of the adsorbent, taking account of the dead volumes, has been determined up to the present.
In other words, numerous papers describe the ranges of values of granulometry and the ranges of values of thickness of the bed, but without establishing the existence of any real relationship between these ranges.
Moreover, certain papers disclose combinations of granulometry and cycle time and/or bed thickness.
Thus, EP-A-0 480 797 teaches a PSA process using adsorbent particles of a diameter 0.4 mm to 1.7 mm, and a cycle time of at least 20 to 60 seconds.
Furthermore, U.S. Pat. No. 4,194,892 discloses an adsorbent bed thickness of at most 2.4 m, adsorbent particles of a diameter at most equal to 0.9 mm and a cycle time of at: least 30 seconds, whilst U.S. Pat. No. 4,194,891 describes an adsorbent bed thickness at most equal to 1 m, adsorbent particles of a diameter at most equal to 0.9 mm and a cycle time of less than 18 seconds.
Furthermore, U.S. Pat. No. 5,071,449 describes an adsorbent bed of a thickness 0.15 m to 1 m, adsorbent particles of a diameter 0.2 mm to 1 mm and a cycle time of 6 seconds to 60 seconds, and the document "Gas Separation by Adsorption Processes", Yang, 1989, p. 267, discloses an adsorbent bed thickness of 1.8 m to 3 m, adsorbent particles of a diameter equal to 1.6 mm and a cycle time of 3 minutes to 4 minutes.
It will be noted that, for a same diameter of particles, the bed thickness can vary considerably from one paper to another, without actually giving a precise choice and without taking into consideration the geometrical constraints imposed by the dead volumes within the adsorber.
Furthermore, it is known that adsorbents used in PSA units are expensive and that it is thus necessary and desirable to reduce the quantities of adsorbent used, hence the thickness of the adsorbent beds, so as to reduce the cost of production of oxygen.
However, this reduction of adsorbent bed thickness must not impinge on the performance of the PSA unit, particularly the output and the productivity of said PSA unit.